1. Field of the Invention
The present invention relates to an active-matrix liquid crystal display having a display screen provided with a plurality of pixel electrodes, thin-film transistors (hereinafter referred to as xe2x80x9cTFTsxe2x80x9d) as switching elements respectively corresponding to the pixel electrodes, and storage capacitors respectively corresponding to the pixel electrodes.
2. Description of the Related Art
The configuration of a conventional active-matrix liquid crystal display will be described with reference to FIGS. 7 and 8. FIG. 8 is a plan view of an essential part of an array substrate underlying a liquid crystal included in the active-matrix liquid crystal display, and FIG. 7 is a sectional view taken on line 7xe2x80x947 in FIG. 8 and showing the essential part of the array substrate, a liquid crystal layer and an arrangement overlying the liquid crystal layer. As shown in FIG. 7, a liquid crystal layer 31 is filled and sealed in a space between a transparent lower substrate 32 and a transparent upper substrate 33 disposed opposite to the lower substrate 32. Polarizing plates 34 and 35 are attached to the respective outer surfaces of the lower substrate 32 and the upper substrate 33, respectively. Transparent pixel electrodes 36 are formed on the inner surface of the lower substrate 32, and common electrodes 37 are formed on the inner surface of the upper substrate 33 opposite to the pixel electrodes 36.
Referring to FIG. 8, formed on the lower substrate 32 are a plurality of parallel gatelines 42, i.e., scanning lines, and a plurality of parallel source lines 40, i.e., signal lines, extending perpendicularly to the gate lines 42.
The transparent pixel electrodes 36 are formed in rectangular regions surrounded by the gate lines 42 and the source lines 40, respectively. A TFT 38, i.e., a switching element, formed near the intersection of each source line 40 and each gate line 42. The TFT 38 turns on to apply a data signal voltage to the corresponding pixel electrode 36 and turns off to shut the data signal voltage to the same pixel electrode 36. Thin-film storage capacitors Cs for holding charges on the pixel electrodes 36 are formed on the gate lines 42.
Each of the TFTs 38 includes a source electrode 40a extending from the source line 40, a drain electrode 41, a gate electrode 42a extending from the gate line 42 and a gate insulating film 43. The drain electrode 41 is connected through a contact hole 45 formed in a layer insulating film 44 to the pixel electrode 36. The pixel electrode 36 is connected through a contact hole 46 to the upper electrode 47 of the storage capacitor Cs. The gate line 42 formed on the lower substrate 32 serves as the lower electrode of the storage capacitor Cs. The upper electrode 47 is disposed opposite to the gate line 42 with the gate insulating film 43 interposed therebetween.
In this conventional active-matrix liquid crystal display, a data signal voltage is applied to a selected one of the plurality of source lines 40 and a control signal is applied to a selected one of the plurality of gate lines 42 to drive the TFT 38 connected to the selected source line 40 and the selected gate line 42, whereby the data signal voltage is applied to the pixel electrode 36 connected to the drain electrode 41 of the TFT 38. The TFTs 38 respectively connected to the pixel electrodes 36 arranged in a matrix are thus driven to display a desired pattern on the screen.
As shown in FIG. 7, the gate insulating film 43 of the TFT 38 serves also as an insulating film for the storage capacitor Cs. Use of a single film formed by a single process as both the gate insulating film 43 and the insulating film for the storage capacitor Cs is favorable in view of simplifying a liquid crystal display fabricating process. The thickness of the film is determined so that the gate insulating film 43 of the TFT 38 has a sufficient dielectric strength.
Although dependent on the configuration of the storage capacitor Cs, a voltage to be applied to the storage capacitor Cs is in the range of xc2xc to xc2xd of a voltage to be applied to the TFT 38. Therefore, a thickness of the insulating film for the storage capacitor Cs having a sufficient dielectric strength may be smaller than that of the gate insulating film 43 of the TFT 38. Thus, the thickness of the gate insulating film 43 of the TFT 38 is excessively great for the insulating film for the storage capacitor Cs.
The area of the storage capacitor Cs is calculated by dividing a predetermined charge storage capacity necessary for driving the pixel electrode 36 by the storage capacity per unit area of the thin-film storage capacitor Cs. Since the storage capacity per unit area is uniquely dependent on the dielectric constant and the thickness of the gate insulating film 43, the area of the storage capacitor Cs is determined on the basis of the dielectric constant and the thickness of the gate insulting film 43.
An active-matrix liquid crystal display must have a high optical transmittance, i.e., a large aperture ratio, to display pictures in a high picture quality. When the size of pixels is reduced for high-definition displaying, an area occupied by the storage capacitors Cs increases relatively and hence the aperture ratio is reduced. If the brightness of back light is increased to compensate the reduction of the aperture ratio, the power consumption of the active-matrix liquid crystal display increases.
The area of the storage capacitors Cs needs to be reduced to increase the aperture ratio. However, since the insulating film for the storage capacitors Cs is part of the gate insulating film 43 of the TFTs 38 and the thickness of the gate insulating film 43 is determined on the basis of the dielectric strength of the TFTs 38, the capacitance per unit area of the storage capacitor Cs cannot be individually increased. Consequently, the area of the storage capacitor Cs cannot be reduced securing the predetermined capacitance of the storage capacitor Cs and the aperture ratio decreases inevitably when the size of the pixels is reduced for high-definition displaying.
Accordingly, it is an object of the present invention to provide an active-matrix liquid crystal display having a large aperture ratio achieved by forming storage capacitors each having a small area relative to that of pixel electrodes.
According to a first aspect of the present invention, an active-matrix liquid crystal display comprises: a transparent upper substrate; a transparent lower substrate disposed opposite to the upper substrate; a liquid crystal filled and sealed in a space between the upper and the lower substrate; a plurality of parallel gate lines formed on the lower substrate; a plurality of parallel source lines formed on the lower substrate so as to extend perpendicularly to the gate lines; TFTs formed at intersections of the gate lines and the source lines, respectively; pixel electrodes connected to the TFTs, respectively; and storage capacitors connected to the pixel electrodes, respectively; wherein separate films are used as an insulating film included in the storage capacitors and a gate insulating film included in the TFTs, respectively, and each of the storage capacitors has an upper electrode connected to the pixel electrode and a lower electrode disposed opposite to the upper electrode with the insulating film sandwiched between the upper and the lower electrode. The capacitance per unit area of the storage capacitors can be determined independently of the thickness of the gate insulating film for the TFTs. Therefore, the area of each storage capacitor can be reduced relative to the area of each pixel electrode by increasing the capacitance per unit area of the storage capacitors.
Preferably, the upper electrodes are formed only in flat regions on the insulating film overlying the lower electrodes. When the upper electrodes are thus, formed, dielectric breakdown at steps in the insulating film around the lower electrode does not occur easily. Therefore, the thickness of the insulating film can be reduced so as to meet the required dielectric strength of the storage capacitors, which enables the further increase of the capacitance per unit area of the storage capacitors.
Preferably, the insulating film for the storage capacitors and the gate insulating film for the TFTs are formed of the same material, and the thickness of the insulating film for the storage capacitors is smaller than that of the gate insulating film for the TFTs. A gate insulating film forming process can be applied to forming the insulating film. Thus, the capacitance per unit area of the storage capacitors can be increased and the area of each storage capacitor can be reduced relative to the area of each pixel electrode without complicating an active-matrix liquid crystal display fabricating process.
The insulating film for the storage capacitors may be formed of a material having a dielectric constant greater than that of a material forming the gate insulating film. Thus, the capacitance per unit of the storage capacitors can be increased and the area of each storage capacitor can be reduced relative to that of the pixel electrode.